In recent years, light-emitting devices have included quantum-dot emitting layers to form large area light emission. One of the predominant attributes of this technology is the ability to control the wavelength of emission, simply by controlling the size of the quantum dot. As such, this technology provides the opportunity to relatively easily design and synthesize the emissive layer in these devices to provide any desired dominant wavelength, as well as control the spectral breadth of emission peaks. This fact has been discussed in a paper by Bulovic and Bawendi, entitled “Quantum Dot Light Emitting Devices for Pixelated Full Color Displays” and published in the proceedings of the 2006 Society for Information Display Conference. As discussed in this paper, differently sized quantum dots may be formed and each differently-sized quantum dot will emit light at a different dominant wavelength. This ability to tune emission provides opportunities for creating very colorful light sources that employ single color emitters to create very narrow band and, therefore, highly saturated colors of light emission. This characteristic may be particularly desirable within the area of visual displays, which typically employ a mosaic of three different colors of light-emitting elements to provide a full-color display.
Applications do exist, however, in which it is desirable to provide less saturated light emission and/or highly efficient light emission. An application for highly efficient, broadband light emission exists in general lighting devices. Within this application area there are multiple requirements that such a light source must provide. First, the light source must provide at least one color of light that is perceived to be white. This white light requirement is typically specified in terms of color temperature or coordinates within the CIE 1931 chromaticity diagram. Secondly, the light source must be highly energy efficient. Thirdly, at least in some applications, the light source must be broadband in nature to facilitate color constancy of objects viewed under this lighting source to the same objects when viewed under natural lighting. Within this industry, it is typical to employ light-emitting materials in a single package to form a single light source. For example, typical fluorescent light bulbs employ at least a red, green, and blue phosphor to form the desired color of light emission. Further, OLED light source prototypes have been demonstrated that employ multiple dopants in a single or in multiple layers to form a white light source. However, within these systems, the spectral characteristics of light emission are highly dependent upon the molecular structure of the material that forms the light-emissive layer or the dopant that is applied, and therefore device designers must select among relatively few materials, all of which have different radiant efficiencies and spectral emission characteristics. Therefore, it is difficult to design a device that has ideal characteristics for all applications.
Within the information display application space, devices are desired to deliver a large color gamut with high efficiency. Within this application space, efficiency is typically measured in metrics such as the number of candelas that are produced as a function of input current or power. Therefore, the two requirements of large color gamut and high efficiency are often in conflict with one another. This conflict occurs due to the fact that as the color gamut of the display is expanded, the red and blue emitters must often be shifted towards very short and very long wavelengths, respectively, and the human eye is much less sensitive to these wavelengths than to wavelengths of light near the center of the visible spectrum. This loss of sensitivity to energy at the extremes of the visible spectrum occurs because luminous efficiency, measured in candelas, is calculated by cascading the eye sensitivity function with the radiant power spectrum of light emission. FIG. 1 shows the efficiency function of the human eye, which shows the percent efficiency of the eye to converting energy at each wavelength within the visible spectrum to an increase in perceived brightness. As this figure shows, the human eye is most sensitive to energy with a wavelength of between 550 and 560 nm 2, but much less sensitive to a very short wavelength 4 or very long wavelength 6 within the visible spectrum.
Although the loss of display efficiency that occurs as the color gamut of the display is increased can be largely explained by this discussion of the red and blue emitters, the placement of the green emissive element is also quite important. FIG. 2 shows a 1931 CIE chromaticity diagram having two triangles. The first triangle 8 depicts the color gamut of a display having a green emissive element near 533 nm. The second triangle 10 depicts a larger color gamut that may be achieved by shifting the dominant wavelength of a narrowband green emitter to 525 nm. As is readily visible, the color gamut triangle 10 is significantly larger than the color gamut triangle 8; in fact the areas of the two triangles within this color space are 0.18 and 0.19, respectively. However, referring again to FIG. 1, it may be observed that shifting the green primary from 533 nm to 525 nm, which provides a larger color gamut, reduces the efficiency at which the human eye converts radiant power to perceived brightness from 90% to only 79%.
Numerous methods for improving the overall efficiency of a display device have been discussed in the literature. One such method is to simply select the RGB primaries to provide high efficiency while at the same time providing an “optimal gamut” as suggested by William A. Thornton in a paper entitled “Suggested Optimum Primaries and Gamut in Color Imaging” and published in Color Research and Application, vol. 25, No. 4. In this paper, the author suggests selecting the primaries of the display device to match the “prime colors” for the human visual system. As the authors suggests, this would establish a system having emitters with peak wavelengths of 450, 530, and 610 nm for the blue, green, and red emissive elements, respectively. This approach supposedly allows the display to provide maximum peak brightness for a given input energy, if it is assumed that the radiant efficiency of each of the emitters is equivalent. Unfortunately, this approach limits the color gamut of the display. In fact, the color gamut triangle 8 in FIG. 2 is obtained when the display uses light-emitting elements having these same peak wavelengths, each light-emitting element having a 30 nm bandwidth. Of further concern with this approach is that the red primary is particularly desaturated and the color of this primary may be more accurately described as orange rather than red. Therefore, while the approach described by Thorton does provide a display with good energy efficiency, it would not provide a display with a particularly good visual appearance.
A second method, which has been discussed within the organic light emitting diode art, involves the use of additional, more efficient, primaries to the typical three primary systems. For instance Burroughes in WO/0011728, entitled “Display Devices” describes an OLED system having red, green, and blue light emissive elements and at least one further light emissive element for emitting a color to which the human eye is more sensitive than the emission color of at least one of the red and blue emitters. Unfortunately, Burroughes fails to recognize that in most applications, it is particularly important to render white with high efficiency, a fact that is discussed by Miller et al in US Patent Application US 2005/212728, entitled “Color OLED Display With Improved Power Efficiency”. As this application discusses the optimal power benefit when adding additional narrowband emitters to the display requires the addition of at least two additional light-emitting elements; one for emitting yellow and one for emitting cyan light. Therefore, in devices such as these, which add additional saturated color primaries, it is typically necessary to add at least two additional emitters to achieve the maximum gains in power efficiency. However, the addition of each additional primary increases the manufacturing cost of the display device since additional elements must be formed and patterned to form each colored light-emitting element, requiring more precise patterning technology to allow these additional features to be patterned within the same plane as the original three emitting elements. Image quality of the display is also often sacrificed, as there is a need for a total of 5 emissive elements per pixel, 2 of which will often be inactive at any point in time.
Another approach discussed in the organic light emitting diode literature is to add a single, highly efficient white emitting element to the display device as discussed by Siwinski in U.S. Pat. No. 7,012,588, entitled “Method For Saving Power In An Organic Electro-Luminescent Display Using White Light Emitting Elements”. While disclosures, such as this one discuss the use of a white light-emitting element to improve the efficiency of the display system, they do not provide teaching as to the desired spectra of the white emitter, other than to state that it is broadband or emits a white or in-gamut color. Further, disclosures within this area which do discuss the spectral content of such a white emitter, such as US Patent Application 2006/0105198 by Spindler et al., entitled “Selecting White Point For OLED Devices” which discuss the formation of white light-emitting elements using organic materials with broad emission spectra, typically having a bandwidth of greater than 90 nm. Further, as noted earlier, the characteristics of organic white light emitting elements are limited to the characteristics that are available from organic emissive materials. The broadband response of these materials limits their maximum efficiency as energy emission occurs across a broad bandwidth range, including wavelengths to which the human eye is not particularly sensitive.
There is a need, therefore, to provide a display having a very large color gamut and high luminance efficiency, while providing no more than one additionally colored light-emitting element per pixel.